paper

Detection of Ongoing Mass Loss from HD 63433c, a Young Mini Neptune

arXiv:2106.05273 · doi:10.3847/1538-3881/ac3f3b

Abstract

We detect Lyman absorption from the escaping atmosphere of HD 63433c, a , d mini Neptune orbiting a young (440 Myr) solar analogue in the Ursa Major Moving Group. Using HST/STIS, we measure a transit depth of % in the blue wing and % in the red. This signal is unlikely to be due to stellar variability, but should be confirmed by an upcoming second visit with HST. We do not detect Lyman absorption from the inner planet, a smaller mini Neptune on a 7.1 d orbit. We use Keck/NIRSPEC to place an upper limit of 0.5% on helium absorption for both planets. We measure the host star's X-ray spectrum and FUV flux with XMM-Newton, and model the outflow from both planets using a 3D hydrodynamic code. This model provides a reasonable match to the light curve in the blue wing of the Lyman line and the helium non-detection for planet c, although it does not explain the tentative red wing absorption or reproduce the excess absorption spectrum in detail. Its predictions of strong Lyman and helium absorption from b are ruled out by the observations. This model predicts a much shorter mass loss timescale for planet b, suggesting that b and c are fundamentally different: while the latter still retains its hydrogen/helium envelope, the former has likely lost its primordial atmosphere.

accepted by AJ

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